This invention is concerned with the structure of plastic doublet in combination among a compound lens system composed of plural lenses, especially one of the pair is plastic lens. In the lens system, both lenses are fitted in a fitting surface outside the optical field so that friction prevents slip out and both parts are in contact one another in a plane perpendicular to the optical axis outside the optical field.
In the past plastic lenses have been fitted with all their circumference, just like the case of glass lens.
FIG. 35 shows a sectional view of a compound lens consisting of both plastic lens doublet having common optical axis, and FIG. 36 gives its end view taken in direction of arrows X--X in FIG. 35.
In FIGS. 1 and 2 are injection molded plastic lenses, 3 is a contacting surface of plastic lenses 1 and 2 which are in contact in direction of optical axis outside the optical field, and 4 is a circumferential fitting surface of lens 1 and lens 2, also outside the optical field. The contacting surface 3 is usually consisted of side surface of ring perpendicular to the optical axis, and the fitting surface 4 denotes inner and outer circumferences of cylinders. In conventional way of combination, such compound lens is usually held in a holder, of which outermost diameter is not indicated in the figure, and fixed with a threaded ring against a reamed seat in the holder in the direction of arrows.
Injection-molded plastic lenses are (1) difficult to form strict shape in diameter because material is injected through sprue gate on its circumference, (2) less controllable in the accuracy of diameter than that of lens curvature. (3) Temperature produces more thermal effects on the dimensions and form of plastic lens than that of glass lens. (4) There is need to orient a special radial direction for lessening influence of birefringence on combination of plastic lenses. For the reasons, the conventional way of fitting in which the fitting surface 4 is formed of inner and outer circumference of projecting cylinder as shown in FIG. 35 and 36 is led to some problems: Compound lens of plastic doublet is liable to suffer eccentricity/play of component lens due to its unavoidable tolerance in diameter, or reversely forced fitting produces elastic deformation prevailing inside the optical field, or lens fit itself is not available. In any case, it is difficult to obtain an practical fit free from both eccentricity/play and deformation inside optical field.